1. Field of the Invention
The present invention relates generally to a communication system supporting packet data service, and in particular, to a method and apparatus for transmitting information indicating a spreading code used for a packet data channel (PDCH).
2. Description of the Related Art
A conventional mobile communication system provides only a voice service. Growing user demand and advanced communication technology have developed mobile communication systems that can additionally provide a data service. A mobile communication system supporting multimedia service including voice and data services provides voice service to a plurality of users in the same frequency band and provides data service in time division multiplexing (TDM). Particularly, a CDMA (Code Division Multiple Access) mobile communication system transmits user data to a particular user for a predetermined time by TDM.
The mobile communication system supporting packet data service uses the PDCH that delivers actual packet data and the PDCCH (Packet Data Control Channel) that deliver control information for efficient packet data transmission. Packet data transmission is carried out on a PLP (Physical Layer Packet) basis and a receiver efficiently receives the packet data using the control information.
The control information informs users that intend to receive a data service on the PDCH of a user to which packet data is destined for at a particular time point, its length, and its transmission parameters, such as a modulation scheme and a coding rate used. Fields of the control information on the PDCCH are illustrated in Table 1. It should be noted here that the control information can be formed by selecting part of the fields or by adding additional fields to the fields, and the length of each field varies depending on system implementation.
TABLE 1Field.Length in bitsMAC_ID8ARQ Channel ID2Subpacket ID2Encoder Packet Size3Last Walsh Code Index5Sequence Bit1Total21 
Referring to Table 1, the control information is formed using an 8-bit MAC (Media Access Control) ID (Identifier) for identifying a user, a 3-bit payload size (i.e., a 3-bit encoder packet size), a 2-bit SPID (Subpacket ID) for indicating the number of retransmission occurrences of the same packet data, a 2-bit ARQ (Automatic Repeat Request) channel ID for indicating a channel that delivers packet data, a 5-bit last Walsh code index for indicating Walsh codes used for spreading the PDCH, and 1-bit sequence information.
In a mobile communication system supporting high-rate packet transmission, a subpacket is a transport unit for transmitting data on the PDCH. Subpacket length is the temporal length of TDM data transmitted on the PDCH. If the data length is variable, a changed data length must be represented to a receiver. Since a transmitter repeats the control information according to the subpacket length prior to transmission, the receiver determines the subpacket length from the control information. The data rate and modulation scheme of the PDCH are determined according to a combination of the subpacket length, the payload size, and Walsh codes used for the PDCH. The data rate is the transmission rate of the packet data on the PDCH and the modulation scheme is one of QPSK (Quadrature Phase Shift Keying), 8PSK (8-ary PSK), 16QAM (16-ary Quadrature Amplitude Modulation), and 64QAM.
The payload size is the number of information bits included in one subpacket, the SPID identifies a set of subpackets retransmitted by ARQ, and the ARQ channel ID identifies a parallel transmission channel to support continuous data transmission for the user.
To receive a high-rate packet data service, a mobile station (MS) is assigned to its unique MAC ID at system access and determines whether its packet is received on a PDCH by checking a MAC ID obtained from a PDCCH through demodulation. If a packet is destined for the MS, the MS demodulates the PDCH using the other information fields of the PDCCH, that is, the Payload Size, the SPID, the ARQ Channel ID, and the Last Walsh code Index.
FIG. 1 is a block diagram of a conventional PDCCH transmitter 100. Referring to FIG. 1, an error detection bits adder 110 adds, for example, an 8-bit CRC (Cyclic Redundancy Code) to a 21-bit F-PDCCH (Forward PDCCH) input sequence containing control information to detect transmission errors in the input sequence.
A tail bits adder 120 adds tail bits to the 29-bit CRC-added sequence, for convergence to a predetermined state. The tail bits are 8 zeroes for convolutional encoding. A convolutional encoder 130 encodes the 37-bit information received from the tail bits adder 120 at a predetermined coding rate R. The coding rate R is determined according to the number N of slots that transmit the control information. If the control information requires 1 slot, the convolutional encoder 130 outputs two symbols for the input of every one bit (R=½). If the control information requires 2 or 4 slots, the convolutional encoder 130 outputs three symbols for the input of every one bit (R=⅓).
A symbol repeater 140, if N=4, repeats the convolutional code symbols one time. As a result, the symbol repeater 140 outputs 74, 111, and 222 symbols when N=1, 2 and 4, respectively.
A symbol puncturer 150 punctures 26, 15, and 30 symbols in the repeater output when N=1, 2, and 4, respectively, according to a puncturing pattern that minimizes performance degradation and matches to an appropriate data rate. An interleaver 160 interleaves the punctured symbols according to a predetermined interleaving rule to reduce the influence of burst errors that degrade coding performance. The interleaver 160 can be a kind of block interleaver, such as a BRI (Bit Reverse Interleaver). The BRI arranges even-numbered symbols in a first half of an interleaver output and odd-numbered symbols in a last half, after interleaving such that the distance between symbols is widest.
A modulator 170 modulates the interleaved symbols in a predetermined modulation scheme like QPSK. Spreaders 180 spread I (In phase)-channel modulated symbols and Q (Quadrature phase)-channel modulated symbols with a spreading code Wi64 assigned to the PDCCH. The spread signals are transmitted to an MS.
FIG. 2 is a block diagram of a conventional PDCCH receiver 200 corresponding to the PDCCH transmitter 100 illustrated in FIG. 1. As described above, control information is received in 1, 2, or 4 slots and the number N of the slots is equal to the length of packet data. Therefore, the PDCCH receiver 200 determines the length of the control information, that is, the length of the packet data using a received PDCCH signal.
Referring to FIG. 2, the PDCCH receiver 200 comprises four reception blocks 210 to 240. The four reception blocks 210 to 240 each receive 48 symbols and check the CRC of the received symbols, thereby detecting the packet length. The symbols are demodulated soft decision values.
The first reception block 210 is used to receive 1-slot control information about 1-slot packet data, the second reception block 220 is used to receive 2-slot control information about 2-slot packet data, the third reception block 230 is used to receive 4-slot control information about 4-slot packet data, and the fourth reception block 240 is used to receive 4-slot control information about 8-slot packet data.
In operation, deinterleavers 212, 222, 232, and 242 in the first to fourth reception blocks 210 to 240 deinterleave 48, 96, 192 and 192 symbols received respectively in 1, 2, and 4 slots. Depuncturers 214, 224, 234, and 244 respectively depuncture 10, 20, 40, and 40 symbols in the deinterleaved symbols. Combiners 235 and 245 in the reception blocks 230 and 240 combine every 2 consecutive symbols in the depunctured symbols received from the depuncturers 234 and 244, in an order reverse to the operation of the repeater 140 illustrated in FIG. 1.
Convolutional decoders 216, 226, 236, and 246 decode the depunctured symbols received from the depuncturers 214 and 224 and the combined symbols received from the combiners 235 and 245 at corresponding coding rates. The convolutional decoder 216 for 1-slot control information decodes at a coding rate of ½, and the convolutional decoders 226, 236, and 246, for 2- or 4-slot control information, decode at a coding rate of ⅓.
CRC checkers 218, 228, 238, and 248 check the CRCs of the decoded data using predetermined initial values of 1 (the CRC checkers 218, 228, and 238) and 0 (the CRC checker 248).
A controller 250 determines the length of control information, that is, the length of packet data according to the CRC check results. Specifically, the controller 250 determines the packet data length as the slot length corresponding to a reception block having a good CRC check value.
The four reception blocks 210 to 240 can be constituted as physically independent blocks, or incorporated into a single reception block having different reception parameters including an interleaving rule, a puncturing pattern, a coding rate, and an initial value.
The PDCH is usually spread with all available Walsh codes of length 32. The available Walsh codes are the remaining Walsh codes in a whole code space, not including Walsh codes assigned to circuit channels for voice service and Walsh codes available to common channels. Hence the PDCH spreading codes are variable.
The PDCCH delivers information about the spreading codes of the PDCH, as stated before. A base station (BS) transmits to an MS on the PDCCH the index of the last of the assigned Walsh codes in a Walsh code list preset between the BS and the MS. The MS receives the PDCH by receiving the PDCCH.
FIG. 3 illustrates an example of a Walsh code list. Referring to FIG. 3, a total of 28 Walsh codes of length 32 are available to the PDCH. The BS informs the MS of the 5-bit index of the last of Walsh codes assigned to the PDCH.
If six Walsh codes assigned to the PDCH are #31(index 0), 15(1), 23(2), 7(3), 27(4), and 11(5) are used for the PDCH, the BS transmits the MS the index “00101” (5) of the last Walsh code #11 on the PDCCH. The MS then extracts Walsh code #11 from the Walsh code list, considering that Walsh codes #31, 15, 23, 7, 27 and 11 have been used for the PDCH.
If Walsh codes having a shorter length, 4 or 8 are used for circuit channels to provide a high-rate circuit data service, it may occur in the above Walsh code information transmitting method that Walsh codes indicated by the index of the last Walsh code cannot be used for the PDCH. This problem is apparent in FIG. 4.
FIG. 4 illustrates Walsh codes assigned to voice and circuit data channels and Walsh codes available to the PDCH. Referring to FIG. 4, the first column is a length 32 Walsh code list, and the second, third and fourth columns are length 16, 8, and 4 Walsh code lists, respectively. A Walsh code of length 4 assigned to a circuit data channel, that is, an F-SCH (Forward Supplemental Channel) is marked with slanting lines from the upper left to the lower right, Walsh codes assigned to the voice service and F-SCHs are marked with horizontal lines, and Walsh codes available to the PDCH are marked with slanting lines from the lower left to the upper right.
Referring to FIG. 4, since a length 4 Walsh code #3 is in use for the F-SCH, the first Walsh code available to the PDCH is in the middle of the length 32 Walsh code list. In this case, although 11 Walsh codes #29 to #26 are available to the PDCH, the transmitter cannot represent the available Walsh codes using the 5-bit last Walsh code index. Consequently, the receiver cannot determine the Walsh codes assigned to the PDCH.